"This is a question of whether or not we want to be in the eurozone," said Papandreou. "That is very clear."

Papandreou made the remark following a difficult meeting with French President Nicolas Sarkozy and German Chancellor Angela Merkel ahead of the Group of 20 Summit in Cannes, France.

Merkel and Sarkozy said Greece will not receive the latest installment of its bailout money, which the nation needs to avoid a default, until after the referendum, which could take place as soon as December 4.

"It's up to the Greeks now to determine whether they want to continue on this road with us or not," said Sarkozy, adding that all European Union members must abide by certain rules.

"If any country wishes to throw out these rules, they are free to do so, but they must leave the eurozone," he said.

Meanwhile, Papandreou said he believes the voters will approve the plan. But he added that the government needs a "wide consensus."

"Greece's position within the euro area is a historic conquest of the country that cannot be put in doubt. The country must feel safe and stable, " said Greek Finance Minister Evangelos Venizelos, upon his return to Athens, following the Cannes meeting. "If we want to protect the country we must ... implement without any delay the decision of October 26 [sic]. Now, as soon as possible."

Early Thursday morning, Papandreou called an emergency cabinet meeting.

Concern about how the referendum would affect the financing of Greece's bailout rattled Wall Street Wednesday.

"As soon as the referendum is completed, and all uncertainty removed, I will make a recommendation to the IMF Executive Board regarding the sixth tranche of our loan to support Greece's economic program," said IMF managing director Christine Lagarde.

Merkel described the talks as "difficult" but she said the discussions were conducted in a "spirit of partnership."

She said finance ministers from Germany, France and the 17 other members of the eurozone will meet Thursday to discuss ways to accelerate implementation of the latest crisis measures.

In addition to the debt relief and bailout for Greece, EU leaders agreed last month to boost capital requirements for banks and leverage a government-backed rescue fund to prevent the debt crisis from spreading to Italy.

The details of the so-called comprehensive plan were expected to be outlined here as the world's most powerful political leaders gather for talks on the global economy.

But the crisis in Greece has dominated the G20 meeting after Papandreou announced the referendum on Tuesday.